The present invention relates generally to the HIPing of composite structures. More particularly it relates to improvement in the HIPing of a composite structure in which a grid of reinforcing elements is embedded within a metal matrix.
The present invention is directed particularly to improving the product formed by HIPing a composite structure in which fine filaments of reinforcement are embedded within a matrix of metal which has been deposited by plasma spray technique.
The formation of composite structures having filaments of reinforcing material such as aluminum oxide fibers or silicon carbide fibers embedded within a metal matrix, such as a titanium base matrix, have been disclosed in the art.
The preparation of titanium alloy base foils, sheets, and similar articles and of reinforced structures in which silicon carbide fibers are embedded in a titanium base alloy are described in U.S. Pat. Nos. 4,775,547; 4,782,884; 4,786,566; 4,805,294; 4,805,833; and 4,838,337; assigned to the same assignee as the subject application. The texts of these patents are incorporated herein by reference. Preparation of composites as described in these patents is the subject of intense study inasmuch as the composites have very high strength properties in relation to their weight. One of the properties which is particularly desirable is the high tensile properties imparted to the structures by the high tensile properties of the reinforcing fibers or filaments. The tensile properties of the structures are related to the rule of mixtures. According to this rule the proportion of the property, such as tensile property, which is attributed to the filament, as contrasted with the matrix, is determined by the volume percent of the filament present in the structure and by the tensile strength of the filament itself. Similarly, the proportion of the same tensile property which is attributed to the matrix is determined by the volume percent of the matrix present in the structure and the tensile strength of the matrix itself.
Prior to the development of the processes described in the above-reference patents, such structures were prepared by sandwiching the reinforcing filaments between foils of titanium base alloy and pressing the stacks of alternate layers of alloy and reinforcing filament until a composite structure was formed. However, that prior art practice was found to be less than satisfactory when attempts were made to form shaped structures in which the filament was an internal reinforcement for the entire shaped structure.
The structures taught in the above-referenced patents and the methods by which they are formed, greatly improved over the earlier practice of forming sandwiches of matrix and reinforcing filament by compression.
Later it was found that while the structures prepared as described in the above-referenced patents have properties which are a great improvement over earlier structures, the attainment of the potentially very high ultimate tensile strength of these structures did not measure up to the values theoretically possible. The testing of the composites formed according to the methods taught in the above patents has demonstrated that although modulus values are generally in good agreement with the rule of mixtures predictions, the ultimate tensile strength is usually much lower than predicted by the underlying properties of the individual ingredients to the composite. A number of applications have been filed which are directed toward overcoming the problem of lower than expected tensile properties and a number of these applications are copending. These include applications Ser. No. 445,203 pending November 13, filed Dec. 4, 1989; Ser. No. 459,894, filed Jan. 2, 1990, U.S. Pat. No. 4,978,585 and Ser. Nos. 455,041 and 455,048 pending November 13 both filed Dec. 22, 1989. The texts of these applications are incorporated herein by reference.
One problem which we have encountered in forming complex structures is the problem of filament breakage. Some of the patents referenced above, and particularly the U.S. Pat. Nos. 4,786,566 and 4,782,884, teach a method by which monotapes can be formed. Such monotapes constitute a planar structure in which an array of parallel reinforcing filaments are embedded in a titanium base matrix metal. According to the patents referenced above, the embedding of the reinforcing filament within the titanium base matrix is accomplished by plasma spray depositing the titanium base matrix about the filaments. Such plasma spray depositing has been very effective in embedding the filament because the droplets of spray are essentially liquid at the time they contact the filament reinforcement and damage to the filaments in a formation of the single ply monotapes is accordingly minimal. However, when a number of such tapes are mounted together and a HIPing consolidation is applied thereto, there is a tendency toward breakage of the embedded filament. Primarily it is thought because of the irregularities of the surface of the spray deposited titanium base matrix metal. Such irregularities are thought to generate point contact stresses in embedded filaments and to cause cracking and other damage thereto. The irregularities at the surface of the plasma spray deposited matrix metal is a by-product of the plasma spray process itself. It is therefore an advantage to employ the as-deposited matrix in forming composite structures because of the lower cost of employing the monotapes in the as-formed condition.
We have found that the damage to the embedded fibers occurs principally when planar multilayer structures are formed by compression or by HIP consolidation.
The HIP consolidation of composite structures is a well known process and its practice has been applied to many different structures. As noted above, it can be employed in the fiber and foil type of consolidation as well as in the consolidation of a product which is initially solid and coherent to increase the density of the composite and to reduce the proportion of voids therein.
As noted above, one of the problems which is associated with formation of filament reinforced metal structures which are prepared by methodologies which include a HIPing consolidation step is the problem of filament breakage. The filaments which are employed to strengthen composite metal products are generally high strength ceramic base products. These products have extremely high tensile properties along their length, but because they are ceramic in nature, they are highly subject to breakage due to point contact stresses and strain. A simple way in which a determination can be made of the effectiveness of processing steps in preserving the integrity of elementary reinforcement and in reducing the degree to which such filamentary reinforcement are subject to breakage is by dissolving away the matrix metal following the processing in order to determine by visual observation the degree to which the reinforcing filament has been cracked and broken. It has been observed from such testing that a significant part of the breakage which occurs, occurs as the composite structure is consolidated by an action such as the HIPing action.